1. Field of the invention
The present invention relates to a mullite sintered body, a method for producing the same, and a composite substrate.
2. Description of the Related Art
A mullite sintered body is a material superior in resistance to thermal shock, produced by sintering aluminum oxide (Al2O3) and silicon oxide (SiO2) in a proportion of 3 to 2 and is expressed by 3Al2O3.2SiO2. For example, PTL 1 discloses a mullite sintered body produced by compacting a powder prepared by mixing 30% by mass of yttria-stabilized zirconia (YSZ) powder to mullite powder, and sintering the compact. In PTL 1, the mullite sintered body is cut into a mullite substrate, and the main surface of the mullite substrate is polished for use as a base substrate to be bonded to a GaN substrate. It is believed that the thermal expansion coefficients of GaN and mullite are 6.0 ppm/K and 5.2 ppm/K, respectively, in a temperature range from room temperature to 1000° C. In view of bonding these substrates together for use, it is preferable to increase the thermal expansion coefficient of mullite so as to be close to the thermal expansion coefficient of the GaN substrate. For this sake, YSZ powder is mixed to mullite powder, and the mixture is sintered.